1. Field of Use
The present invention relates to the field of wireless communications by means of electromagnetic signals in an underwater environment.
2. Description of the Related Art
The field of underwater wireless communications is of key importance in a range of industries: oil and gas exploration; environmental monitoring; security and defense. The majority of underwater wireless systems are based on the propagation of mechanical waves, in the form of sonar, or acoustic signaling.
U.S. Pat. No. 6,130,859, “Method and Apparatus for Carrying out High Data Rate and Voice Underwater Communication”, Sonnenschein et al. describes a system for underwater communications based on the transmission and reception of acoustic waves. Sonnenschein describes a system where electrical signals are converted to mechanical signals for underwater transmission and vice versa using hydrophones. Communications systems based on the propagation of acoustic or mechanical waves, such as those taught in Sonnenschein suffer from a range of drawbacks. These drawbacks include distortion due to multi-path effects, echoes, Doppler effects, the long time delay between source and receiver, and the lack of a means to discriminate between signals which are produced by several sources.
Advances have been made in the area of wireless telecommunications using electromagnetic signals over the last few decades; however, the recent technological advances apply almost exclusively to wireless communications in air. Wireless communications using electromagnetic signals would be a preferred means for underwater communications. Electromagnetic signals can be produced by a transmitter using well established radio circuitry and design. Electromagnetic signals can be divided into multiple channels, and data modulated onto each channel separately. A vast range of protocols are available for the processing of data sent by electromagnetic signals. Radio transmitters are efficient and do not produce the unwanted drawbacks of acoustic wireless systems.
Unfortunately, electromagnetic signals and radio waves are severely affected by the high conductivity of water (especially sea water) which produces a very high level of attenuation with distance for an underwater radio signal. This effect limits the use of electromagnetic signals for underwater communications to short-range low-bandwidth communications.
GB Patent Application No 8420017, “Inductive Communications System” Raynor, describes a method for underwater communications within a short range by exciting the magnetic component of an electromagnetic signal. However, to overcome the effects of attenuation under water, the inductive communications system taught by Raynor is implemented using low frequency carrier signals. The use of low frequency carrier signals for in wireless communications presents limitations for channel capacity as taught by C. E. Shannon in “Communication in the presence of noise” Proc. Institute of Radio Engineers vol. 37 (1): 10-21 (January 1949). Thus, the inductive communications system taught by Raynor is not suitable for the provision of high data transfer rate underwater wireless communications.